1. Field of the Invention
The present invention relates to a pickup apparatus, a recording and reproduction apparatus, and a recording and reproduction method. In particular, the present invention relates to a pickup apparatus, a recording and reproduction apparatus and a recording and reproduction method suitable for the recording and the reading of information relative to an optical recording medium such as an optical disc.
2. Description of the Related Art
The recording or the reading of information on an optical recording medium such as an optical disc is commonly performed by irradiating the optical recording medium with light from a light source to form or to read a minute change in reflectance on one side of the optical disc.
However, recently, there has been further progress toward an increase in the amount of content able to be stored, and a higher level of precision and complexity, whereby storage of a large amount of content on such a recording medium has been promoted.
For this reason, there is a necessity to reduce a spot diameter of an illumination light used in the recording and the reproduction of information. Furthermore, it is known that the size of the optical spot on the disc is approximately given by λ/NA (λ: the wavelength of the illumination light, NA: numerical aperture) and the resolution is also proportional thereto.
For example, in Y. Kasami, Y. Kuroda, K. Seo, O. Kawakubo, S. Takagawa, M. Ono, and M. Yamada, Jpn. J. Appl. Phys., 39, 756 (2000), it is disclosed that a Blu-ray Disc (trade mark) corresponding to about 25 GB is realized by a disc having a diameter of 12 cm, using a blue violet semiconductor laser having the wavelength of 405 nm as a light source, and an object lens having a numerical aperture of 0.85.
In addition to the miniaturization of the pit in a plane direction of the optical disc, recently, the promotion of further improvements in mass storage is being attempted by performing the recording even in the depth direction of the optical disc. For example, in I. Ichimura et al, Technical Digest of ISOM'04, pp. 52, Oct. 11-15, 2005, Jeju, Korea, a method is described for performing high capacity storage by forming a plurality of recording layers in the depth direction of the optical disc.
Furthermore, in S. Kobayashi et. al., Technical Digest of International Symposium on Optical Memory 2009, Th-I-01, Oct. 8, 2009, Nagasaki Brick Hall, a void is formed in a recording layer of one layer by the illumination of laser light to change the reflectance. In addition, a mode of a rose type micro reflector is proposed that performs multilayer recording in a pseudo manner by forming a plurality of voids within the same recording layer even in the depth direction.
In the case of performing the recording and the reproduction on the multilayer optical disc as described above, since distances (hereinafter, referred to as cover thickness) from the disc surface to each recording layer and void are different from each other, a spherical aberration as shown in the following formula (1) is generated in the optical spot.
                    Formula        ⁢                                  ⁢        1                                                                      W          40                =                              1                          5                                ⁢                      1            6                    ⁢                      1            8                    ⁢                                                    N                2                            -              1                                      N              3                                ⁢          N          ⁢                                          ⁢                      A            4                    ⁢                                    t              λ                        ⁡                          [                              λ                ⁢                                                                  ⁢                rms                            ]                                                          (        1        )            
N is a refractive index of the disc and is set to, for example, 1.6. In addition, in the same manner as the example of the Blu-ray Disc, when the wavelength of the laser light source λ=405 nm and the numerical aperture of the object lens NA=0.85, for example, the aberration at a position where the distance from the disc surface is 100 μm becomes 0.46 λrms.
This value is significantly larger than 0.07 rms which is the standard of Marechal known as the index of a diffraction limitation capability, whereby it is an obstacle to performing the multilayer recording and reproduction.
As a method of correcting the spherical aberration, for example, the following methods can be used.    1. A method of using a liquid crystal device    2. A method of generating the aberration by changing the use magnification of the object lens    3. A method of generating the spherical aberration in a relay lens system
The first method uses a liquid crystal having the anisotropy in the permittivity and the refractive index. By applying a voltage to an electrode of the liquid crystal, the refractive index is changed. For this reason, with respect to light penetrating the liquid crystal, a change in phase depending on the electrode pattern can be given.
However, in the case of the multilayer disc with more than 10 layers, the recording layer or the void is formed at a position where the distance from the disc surface is large. Thus, there is a necessity to correct very large spherical aberration.
Furthermore, the change given by the liquid crystal as described above is a step-shaped phase change. For this reason, when correcting the large spherical aberration, since, if the change in phase is not continuously accessed, the remaining aberration is not negligible, the electrode pattern should be formed in a detailed and complicated manner.
Moreover, in the second method, if a reverse spherical aberration is generated by changing the magnification to erase the spherical aberration, the aberration on the axis can be corrected. Thus, normally, this method is often used.
However, in the recording and the reproduction of the optical disc, there is a necessity to perform a transverse shift of the object lens to follow the eccentricity of a recording track. In particular, in the case of using the object lens that realizes the wide spherical aberration correction function, it is easy for a coma aberration generated due to the transverse shift to increase.
On the other hand, in a case where light incident to the object lens is parallel light, even if the object lens is subjected to the transverse shift, the coma aberration is not generated. However, to do that, the light incident to the object lens should be configured so as to become parallel light irrespective of the change in magnification. For this reason, the apparatus becomes more complicated and larger.
Furthermore, in the third method, a reverse spherical aberration is generated in a relay lens system. That is, the spherical aberration generated by a difference in cover thickness up to the recording layer or the void is corrected so as to be erased by the spherical aberration of the relay lens system.
However, in the case of this correction method, when the object lens is subjected to the transverse shift, the spherical aberration of the relay lens system, which was just enough to cancel out, deviates from the spherical aberration which is generated by the difference in cover thicknesses, whereby the comma aberration is generated.